The rapidly increasing adoption of mobile communications facilities, such as mobile telephone systems, has led to a growing demand for the associated user devices, such as mobile telephone handsets and other mobile stations (MS), and for the equipment comprising the network infrastructure, such as base stations. Avoidance of out of band emissions from transmitter circuitry in these devices and equipment is important both for the quality of service provided by the devices and for minimising potential interference caused to other users of the system. To this end, it is essential that the out of band emissions of every device are carefully measured by the manufacturer before shipment to ensure the device is compliant with prescribed limits on the strength of such emissions. Equally however, the manufacturers desire that these tests should be accomplished as quickly as possible, to minimise delays they introduce into production timescales.
In the case for example of equipment intended for use in so-called 3rd Generation mobile communications systems, using code division multiple access (e.g. W-CDMA and cdma2000 systems), the strength of out of band emissions is compared to a “spectrum emission mask” defining maximum acceptable emissions at various offsets from the intended transmitter carrier frequency. The test is specified in terms of a measurement of the CDMA carrier strength, and measurements of multiple (e.g. 4 or 5) frequency bands offset either side of that carrier frequency (see for example preliminary 3GPP Technical Specifications 25.141 and 34.121). The start and stop frequencies and resolution bandwidth are specified for each offset measurement.
One conventional approach to implementing this test involves using a spectrum analyser to measure each offset frequency band individually. This requires the analyser's settings (start frequency, stop frequency, resolution bandwidth and sweep time) to be set up appropriately for each offset, in accordance with the relevant specification. Each change of settings between successive offset measurements requires a small but finite setup time to complete, to input the required settings into the analyser and to allow the analyser to modify its internal operation accordingly. The large number of changes of setting required for the total number of offsets defined in the specifications results in a large cumulative setup time, and therefore a test which is slow overall.
Another known approach to the spectrum emission mask measurement is to use a vector analyser to measure individually successive segments of each offset frequency band, the frequency range of input signals for a segment being restricted to equal the resolution bandwidth. However, this involves performing a large number of complex Fourier transform computations on time-domain data, to generate the required frequency-domain result. The time required the accomplish these computations is generally unacceptably long for manufacturing test purposes.
It is an object of this invention to reduce overall test time for spectrum emission mask and similar measurements.